This application claims priority from R.O.C. patent application Ser. No. 090120147, filed Aug. 16, 2001, the entire disclosure of which is incorporated herein by reference.
The present invention relates to a method for determining an exposure time of a wafer photolithography process, and more particularly to a method for determining the wafer exposure time by analyzing the exposure times of at least previous three batches of production wafers, to reduce the effects of parameter perturbations of facilities and materials.
In semiconductor manufacturing, the photolithography process is an important step, which can include several procedures of dehydration baking, priming, photoresisting, soft baking, exposing, post exposure baking, developing and hard baking, etc. Among those procedures, the exposure procedure is the subject matter of this invention.
The purpose of wafer exposure in the art is to have the photoresist layered over the surface of the wafer to undergo an effective photochemical transformation after absorbing adequate energy, and thereby to enable the developed photoresist to accurately transfer the pattern on the mask onto the wafer for being ready to perform a subsequent etching process. Two of the major operation conditions for the exposure energy control are exposure intensity and exposure time (ET), both of which can directly affect the process yield after wafer etching.
In the wafer photolithography and subsequent etching processes, factors perturbing the wafer exposure energy or the exposure process yield are numerous and can include almost all the operation parameters of each procedure in the entire photolithography process. Some of the crucial factors are photoresist material, photoresist thickness, soft bake extent, development condition, tolerance error of photoresist line width after developed, batch wafer condition, parameter perturbations of machine facilities, and the like. Consequently, in the batch wafer photolithography process, the exposure condition thereof should be adjusted frequently to meet the changes of disturbances or diminutive perturbations which affect the exposure parameters.
In order to control the yield of the photolithography process, it is common to sample the production wafers after completion of one batch photolithography process in the production line for fabricating a test wafer and to take the critical dimension (CD) of the etching line width of the test wafer as a criterion for evaluating the process yield. If the CD value is specs-in (i.e., qualified), the photolithography and etching processes can follow the original exposure conditions. If the CD value is specs-out (i.e., unsatisfied), the photolithography parameters will need to be adjusted (in particular, the exposure time thereof needs to be adjusted). However, in the case that the CD value deviates significantly from the standard, it obviously indicates the need for trouble shooting of the entire photolithography process so as to locate the problem. Regardless of the cause of the perturbations of the system, the yield can usually be adjusted through controlling the exposure time.
To more clearly manifest the time sequential relationship between the inspection line for examining the test wafer and the production line of the production wafer for the photolithography process as a function of time, FIG. 1 shows a schematic diagram of the time sequential relationship of the wafer photolithography production line and the test wafer inspection line among batches.
As shown in FIG. 1, the block L(N) in the production line denotes the photolithography process for the N batch wafer and the block T(N) in the inspection line denotes the examination process for the N batch wafer. The time axis extending from the left side of FIG. 1 to the right side thereof represents the time evolution. As shown in FIG. 1, the exposure condition, on which the manufacture of the L(N) batch production wafer in the production line is based, comes from the T(Nxe2x88x922) batch test wafer. The T(Nxe2x88x922) batch test wafer is sampled from the L(Nxe2x88x922) batch production wafer.
In the above-mentioned process time sequence, while the T(Nxe2x88x922) batch test wafer is under process examination in the inspection line, the photolithography process of the L(Nxe2x88x921) batch production wafer in the production line is simultaneously progressing under the exposure condition on basis of the examination from the T(Nxe2x88x923) batch test wafer. Meanwhile, as seen in FIG. 1, the T(Nxe2x88x923) batch test wafer is sampled from the L(Nxe2x88x923) batch production wafer.
The aforesaid method for determining the wafer photolithography exposure condition is carried out in the manner of xe2x80x9cspacing one batch production wafer.xe2x80x9d The feature of such a method resides in that, for the system composed of the production line and the inspection line, two independent groups are formed: an odd-batch group (including the production wafers and the test wafers of each batch linking with a center line in FIG. 1) and an even-batch group (including the production wafers and the test wafers of each batch linking with a dotted line in FIG. 1).
Nevertheless, this method has at least the following drawbacks. First, when the test wafer CD value is used to judge the input value of exposure time for the batch production wafers in the identical group, the problem of response delay would occur. In the operation of the two independent groups, when one batch test wafer in one group is examined and determined to require large-scale adjustments upon the exposure condition, the adjustments cannot be instantly applied to the other group. In other words, such an approach cannot immediately determine whether the batch production wafer of the other group needs the corresponding adjustments upon the exposure time so as to respond to the possible influences by perturbations of the system parameters. For example, when the L(Nxe2x88x922) batch production wafer in the even-batch group is found to have larger system perturbations from examining the T(Nxe2x88x922) batch test wafer, the adjustments to the exposure time are made on the L(N) batch production wafer. However, the L(Nxe2x88x921) batch production wafer in the odd-batch group can not be suitably reflected in time. Actually, it is not until after the T(Nxe2x88x921) batch test wafer has been determined to need adjustment that the adjusted exposure time can be reflected in the L(N+1) batch production wafer. Under such an arrangement, at least two batches of production wafers (i.e., from L(Nxe2x88x922) batch to L(Nxe2x88x921) batch in this example) will be affected when system perturbations take place.
Second, when the test wafer CD value is used to decide whether or not an adjustment upon the exposure time of the next batch production wafer is required, the problem of adjustment fluctuation of the exposure time would arise. In the operation of the two independent groups, when one batch test wafer in one group is examined to need additional adjustments to the exposure condition, the adjustments are directly made on the production line. Yet, for the other group, such adjustments could become an impulse resulting in an adverse effect upon the production in the group and, empirically, such an impulse may merely be excluded by means of system damping.
For instance, assuming that the L(Nxe2x88x922) batch production wafer in the even-batch group is found to have larger system perturbations from examining the T(Nxe2x88x922) batch test wafer, the adjustment to the exposure time on the production line (which is referred to as xe2x80x9ca first adjustment quantityxe2x80x9d) is primarily applied to the L(N) batch production wafer. Similarly, assuming that the T(Nxe2x88x921) batch test wafer is also examined to need adjustments to the exposure condition, the adjustment made on the production line (which is referred to as xe2x80x9ca second adjustment quantityxe2x80x9d) is related to the L(Nxe2x88x921) batch production wafer which does not undergo the first adjustment quantity, rather than to the L(N) batch production wafer which has undergone the first adjustment quantity. Thus, while the second adjustment quantity is applied to the L(N+1) batch production wafer, the total exposure time adjustment on the production line is the second adjustment quantity (which is the exposure time revision required for the L(Nxe2x88x921) batch production wafer) plus the first adjustment quantity (which has already been applied on the production line). Such an arrangement obviously leads to either insufficient or excessive adjustments to the exposure time, and thus the phenomenon of adjustment fluctuation of the exposure time for the production might occur. Such an adjustment fluctuation is unfavorable to the manufacture of the wafer photolithography process.
Accordingly, regardless of the kind of system variation, at least two batches of wafer production are affected. The crux of the problem resides in that, for the conventional system, it is unable effectively either to integrate the production line as well as the inspection line or to evaluate the trend of the system perturbations in such a way that the system perturbations and the exposure condition input are limited to the characteristics of the two batch groups.
In the case that the parameter or system perturbations are relative small, the process yield is not affected. On the other hand, if the perturbations are substantial, the process yield is materially affected. While the perturbations affect the process yield, the adjustment by either on-line adjustment or shutdown for maintenance is both labor-intensive and costly.
Embodiments of the present invention relate to a method for determining the exposure time of a wafer photolithography process, which can capture the trend of parameter variations of the entire system in the wafer photolithography process by integrating the historical records of the test wafers so as to promptly make adjustments that are desirably conservative adjustments upon the exposure time.
It is another feature of the present invention to provide a method for determining the exposure time of a wafer photolithography process, which can be used prevent the yield of the photolithography process from fluctuation and thus avoid possible reworking by means of applying conservative adjustments of the exposure time so as to effectively solve the problems arising from the use of two independent batch groups in the prior art.
It is a further feature of the present invention to provide a system for determining the exposure time of a wafer photolithography process so as to effectively carry out the method for determining the exposure time of a wafer photolithography process of this invention.
An aspect of the present invention is directed to a method for determining an exposure time of a wafer photolithography process, which is applied to a wafer photolithography system. The system includes a production line and an inspection line. The production line proceeds in turn with the photolithography processes upon a plurality of batch production wafers ( . . . L(Nxe2x88x924), L(Nxe2x88x923), L(Nxe2x88x922), L(Nxe2x88x921), L(N), . . . where N denotes a batch number). The inspection line samples in turn the batch production wafers after completion of the photolithography processes from the production line to fabricate corresponding batch test wafers ( . . . T(Nxe2x88x924), T(Nxe2x88x923), T(Nxe2x88x922), T(Nxe2x88x921), T(N), . . . ). Each of the batch test wafers corresponds to one exposure time for producing a corresponding batch production wafer. The batch test wafer generates a respective test value for comparison with a qualified examination value. The method for determining the exposure time of the L(N) batch production wafer comprises recalling in a time sequence at least three batches of the batch test wafers prior to the L(N) batch production wafer and obtaining corresponding test values for the at least three batches of the batch test wafers; calculating a mean value of the test values corresponding to the at least three batches of the batch test wafers with a predetermined mathematical model; comparing the mean value with the qualified examination value and determining a margin value between the mean value and the qualified examination value to adjust and generate an appropriate process exposure time; and employing the process exposure time as the exposure time of the L(N) batch production wafer.
The method may further include a sampling criterion so as to remove distinctive numerical errors caused by any human or non-human factors. For example, recalling the at least three batches of the batch test wafers comprises, when the difference of the test value corresponding to one of the batch test wafers from the qualified examination value goes beyond a predetermined tolerance, excluding the batch test wafer from the at least three batches. Furthermore, the method may include a function of alarming, which is able to instantly notify on-site operators while the system suffers a distinctive error so that the operators can proceed with the trouble shooting to locate the cause of such a distinctive error and may immediately proceed with either maintenance of the system or revision of parameters so as to avoid further propagation of the unfavorable influences. For example, when the difference of the test value corresponding to the batch test wafer from the qualified examination value going beyond another predetermined tolerance is detected, an alarm signal is generated.
In specific embodiments, the predetermined mathematical model may be a calculation model of an arithmetical mean, a calculation model of a geometrical mean, a calculation model of a weighting average, or some other models. Recalling the at least three batches of the batch test wafers prior to the L(N) batch production wafer comprises recalling the T(Nxe2x88x923), T(Nxe2x88x924) and T(Nxe2x88x925) batch test wafers. The appropriate process exposure time may be adjusted and generated by adjusting the exposure time used to produce the L(Nxe2x88x921) batch production wafer by the margin value. The appropriate process exposure time may be adjusted and generated by adjusting the exposure time used to produce the L(Nxe2x88x922) batch production wafer by the margin value. The appropriate process exposure time may be adjusted and generated by obtaining an average exposure time of the exposure times respectively corresponding to the test values of the at least three batches of the batch test wafers with the predetermined mathematical model, and adjusting the average exposure time by means of the margin value and to generate the process exposure time.
In accordance with another aspect of the invention, the method for determining the exposure time of the L(N) batch production wafer comprises comparing the test value corresponding to each of the batch test wafers with a qualified examination value and determining a margin value between the test value and the qualified examination value to adjust the exposure time used for the batch production wafer corresponding to the batch test wafer to be an estimated exposure time; recalling in time sequence at least three batches of the batch test wafers prior to the L(N) batch production wafer and obtaining the corresponding estimated exposure times; calculating a mean value of the estimated exposure times corresponding to the at least three batches of the batch test wafers with a predetermined mathematical model; and employing the mean value as the exposure time of the L(N) batch production wafer.
Another aspect of the present invention is directed to a system for determining an exposure time of a wafer photolithography process, which is applied to a wafer photolithography system. The system for determining the exposure time of the wafer photolithography process comprises a qualification determining unit, which is connected to the inspection line and is configured to receive the test value and the exposure time corresponding to each of the batch test wafers and to compare the test value with a qualified examination value. A storing unit is connected to the qualification determining unit and is configured to store the test value and the exposure time corresponding to the batch test wafer. A parameter determining unit is connected to the storing unit and is used to recall in time sequence and obtain the test values and the exposure times corresponding to at least three batches of the batch test wafers stored in the storing unit, to determine a process exposure time from the test values and the exposure times with a predetermined mathematical model, and then to transmit the process exposure time to the production line for employment.
In some embodiments, the storing unit is configured to store the test value and the exposure time corresponding to the batch test wafer whose test value differs from the qualified examination value within a predetermined tolerance. An alarming unit is connected to the qualification determining unit and is configured to generate an alarm signal when detecting that the difference of the test value of the batch test wafer from the qualified examination value goes beyond another predetermined tolerance.